The present invention relates to a brake system, and more particularly to a method, apparatus and program product for controlling the release of a parking brake.
Most vehicle designs incorporate parking brakes. Typical parking brake configurations continuously employ regular drum brakes on a rear wheel. Parking brakes commonly rely on simple mechanical linkage to engage the brakes. When engaging an electronically-actuated braking system, an actuator may pull a steel brake cable taut in response to an operator depressing a pedal, lever or button. The resultant tension on the cable is transferred to the end of a brake lever. Other cables may draw brake shoes firmly against the drums in response to the lever""s rotation. As such, the brake shoes restrict vehicle movement.
During release, depression of a knob or button causes the actuator to return to its initial position. The movement of the actuator reintroduces slack into the cables and, consequently, disengages the brake shoes. As such, the success of a brake release operation may depend largely upon the manner in which the actuator releases tension in the brake cable. For instance, should the actuator travel too far, too much cable may be released. This condition requires the actuator to reel in excessive cable slack prior to re-engaging the parking brake, translating into system delays and damage. Conversely, if the actuator does not travel far enough, then too little of the cable will be released. This condition may prevent the brake pads from fully releasing, resulting in wheel drag. Drag may retard vehicle performance, while causing overheating and damage to brake pads and other system components.
To reduce the occurrence of over-release and drag, some brake manufacturers incorporate brake system features that use periodic position adjustments to regulate cable release. For instance, a position adjuster proximate an actuator may periodically retract cable slack to compensate for lining wear and stretching. Another position-based release mechanism consistently moves the actuator a set distance that ideally corresponds to an optimum brake release point. The set distance may manually be adjusted at service intervals to compensate and accommodate for system wear factors. In practice, however, cable stretching and other aging factors cause release points to substantially migrate in between adjustments, promoting drag and over-release.
Other manufacturers rely exclusively on the force-sensing mechanisms to address over-release and wheel drag. Such a system conventionally causes the actuator to travel until a sensor indicates that no load is incident on the brake cable. The position reached by the actuator when zero force is sensed theoretically coincides with an optimum release point, i.e, no drag or over-release. However, errors inherent to the sensors used in these systems tolerate brake system error. For example, a transducer may register zero pounds of force on a cable that actually carries an eight pound load. Such a scenario results in the system halting the release of the cable before the brake shoes disengage. Consequently, there is a need for a brake release mechanism operable to reduce the occurrence of both over-release and wheel drag.
The present invention provides an apparatus, program product and method for controlling the release of an electronically activated parking brake mechanism. A controller may actively monitor a force that is incident on a brake cable element. Program code, executed by the controller, may initiate actuation of the brake cable element. The element may continue to move until a preset load level is indicated. Coincident with the detection of the preset load, the controller may record the position of the brake cable element. A command from the controller may then cause the brake cable element to travel an additional increment. Ultimately, the brake cable element may traverse to a release point that is determined as a function of at least a stored position and the additional increment. The controller may then initiate a second force measurement to ensure the absence of a load on the brake cable element.